This invention pertains to an improved flow distribution grid plate assembly and method used for providing uniform upward fluid flow distribution in ebullated bed catalytic reactors. It pertains particularly to a staged flow distribution grid plate assembly having an upper primary grid plate and a lower secondary grid plate located and supported below the primary grid plate, each plate containing multiple vertical flow tubes.
In ebullated catalyst bed reactors operated at elevated temperature and pressure conditions, flow maldistribution problems sometimes exist below the distribution grid plate and in the catalyst bed above the grid plate. Such flow maldistribution is usually due to abnormal operating conditions such as plugging of openings in the grid plate by coke, or to excessive coke deposits on the catalyst particles in the bed. If such plugging of openings in the grid plate occurs, non-uniform flow distribution and bed ebullation results, which is very undesirable. The riser flow tubes and slotted tail pipes as now used in reactor grid plates usually perform adequately in distributing the recycle and feed liquid streams and hydrogen gas into the ebullated catalyst bed. However, the presently used grid plate arrangement has been found to be inadequate for handling severe flow maldistribution in the plenum of the reactor, for it can only moderately improve the flow distribution existing below the plate, but cannot alleviate "spouts" and major operational upset conditions which lead to an uneven depth of hydrogen in the plenum chamber, which can cause a greater length of the tail pipe slot to be exposed with a corresponding increase in hydrogen flow to those particular riser tubes. Such maldistribution flow conditions in a reactor plenum can be more or less constant depending on the manner in which the feed streams and recycle streams are introduced into the plenum. Also, flow maldistribution could possibly occur as a sloshing effect where the liquid level in the plenum below the distribution grid is constantly tilting from one direction to another.
The use in such ebullated bed catalytic reactors of conventional cylindrical riser flow tubes covered by cylindrical-shaped bubble caps is disclosed by U.S. Pat. No. 3,197,286 to Farkas et al; U.S. Pat. No. 3,197,288 to Johanson, and U.S. Pat. No. 3,475,134 to Weber et al. However, it has been found that inadequate distribution of the gas and liquid flows are usually provided by these reactor designs.
Accordingly, improvements in flow distribution in ebullated bed catalytic reactors have been sought. An improved staged grid plate configuration has now been developed which more effectively redistributes the gas and liquid flows below the primary grid plate whenever flow maldistribution problems exist below the grid plate, so as to provide more uniform ebullation of the catalyst bed in the reactor. To aid in maintaining a "smooth" liquid level in the plenum and consequently a reasonably equal length of slot exposure on each riser tube for gas flow into each tube, a secondary grid plate is provided below the primary grid plate. This secondary grid plate is similar to the single flow distribution grid presently used in ebullated bed reactors, however, the secondary grid does not have caps over the riser tubes on the upper side of the plate, but uses only slotted tubes attached to a plate which extends to near the inner walls of the reactor.